Typically, gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a blade assembly for producing power. Combustors often operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose blade assemblies to these high temperatures. As a result, blades must be made of materials capable of withstanding such high temperatures. Blades and other components often contain cooling systems for prolonging the life of the blades and reducing the likelihood of failure as a result of excessive temperatures.
Blades typically extend radially from a rotor assembly and terminate at a tip within close proximity of the blade rings (in the compressor section) or ring segments (in the turbine section). In the turbine section, the ring segments are mounted to the blade rings and may be exposed to the hot combustion gases and, similar to the blades, the ring segments often rely on internal cooling systems to reduce stress and increase the life cycle. The blade rings or ring segments are spaced radially from the blade tips to create a gap therebetween to prevent contact of the blade tips with the blade rings as a result of thermal expansion of the blades. During conventional startup processes in which a turbine engine is brought from a stopped condition to a steady state operating condition, blades and blade rings pass through a pinch point at which the gap between the blade tips and the blade rings is at a minimal distance due to thermal expansion. The blade tips of many conventional configurations contact or nearly contact the blade rings. Contact of the blade tips may cause damage to the blades. Furthermore, designing the gap between the blade tips and the blade rings for the pinch point often results in a gap at steady state conditions that is larger than desired because the gap and combustion gases flowing therethrough adversely affect performance and efficiency.
As shown in FIGS. 1 and 2, the compressor section 10 of a turbine engine is enclosed within an outer casing 12. The compressor can include a rotor (not shown) with a plurality of axially spaced discs 14. Each disc 14 can host a row of rotating airfoils, commonly referred to as blades 16. The rows of blades 16 alternate with rows of stationary airfoils or vanes 18. The vanes 18 can be provided as individual vanes, or they can be provided in groups such as in the form of a diaphragm. The vanes 18 can be mounted in the compressor section 10 in various ways. For example, one or more rows of vanes 18 can be attached to and extend radially inward from the compressor shell 12. In addition, one or more rows of vanes 18 can be hosted by a blade ring or vane carrier 20 and extend radially inward therefrom.
The compressor section 10 contains several areas in which there is a gap or clearance 22 between the rotating and stationary components. During engine operation, fluid leakage through clearances 22 in the compressor section 10 contributes to system losses, making the operational efficiency of a turbine engine less than the theoretical maximum. Small clearances are desired to keep air leakage to a minimum; however, it is critical to maintain a clearance between the rotating and stationary components at all times. Rubbing of any of the rotating and stationary components can lead to substantial component damage, performance degradation, and extended outages. The size of each of the compressor clearances can change during engine operation due to the difference in the thermal inertia of the rotor and discs 14 compared to the thermal inertia of the stationary structure, such as the outer casing 12 or the vane carrier 20. Because the thermal inertia of the vane carriers 20 are significantly less than the rotor, the vane carrier 20 has a faster thermal response time and responds (through expansion or contraction) more quickly to a change in temperature than the rotor.
Compressor clearance pinch point typically occurs during a hot restart which is a restart of the turbine engine within about thirty minutes after shut down. During the hot restart, the immediate inflow of cool ambient air makes the blade ring contract radially inward faster than the rotor thereby creating the pinch point.
Thus, there is a need for a clearance control system that reduces or minimizes leakage. There is a further need for such a system that avoids contact of the rotating and stationary components.